Noise reducing receiving system



EL H. WEBER, JR

vNOISE REDUCING RECEIVING SYSTEM July 23, 1940.

Filed yJuly 12, 1939 Patented July 23, 1940 l UNITED STATES PATENT OFFICE Application July 12, 1939, Serial 284,087

9 Claims.

"This invention relates to signal receiving circuits and systems,land more particularly to arrangements for use therein to reduce or eliminate the reception of undesired noise currents l and the like. f

The invention has for one of its objects to provide an` arrangement for reducing or eliminating the effect of noise impulses in dependence on the relation that the noise impulses bear with remspect to the strength of the signals Whichrit is desired to receive` l One of the most severe disturbances in signaling systems operating on high andultra high frequencies is the noise produced by the so-oalled man-1nade static much of which results from the ignition systems of automobiles and the like. Such noise can sometimesbe so pronounced to practically entirely masi: the signals to be rc ceived by a receiving circuit or system. Such noise impulses are generally of very short duration, often lasting less than one-thousandth of a second, but the effect of such impulses may last for a longer time. It has been determined that such noise does not seriously interfere Withreception unless the peaks of the noise impulses exceed `the amplitude of the incoming modulated carrier current.

It is one of the features of this 'invention to provide an arrangement for overcoming interference of this type. This may be accomplished by connecting certain special circuits to be described hereinafter in the intermediate radio frequency stages of the radio receiving systermand these special circuits Will act to reduce or elim- "i inate the noise interference from the audio frequency stages of the radio receiver.

This invention Will be better understood from the ldetailed description hereinafter following when read in connection with the accompanying drawing, in which Figure 1 represents one embodiment of the invention for eliminating the noise in the second detector of a radio receiving system. Fig. 2 is a modification of the arrangement of Fig. 1 employed for suppressing the noise 5 from the second detector of the receiving system, and Fig. 3 is a modification of the circuits of Figs. 1 and 2.

Referring to Fig. 1 of the drawing, the rectangle I designates part of the usual radio receiving circuit of the superheterodyne type, which includes an antenna system (not shown) and high frequency and immediate frequency detecting and amplifying apparatus. This apparatus is connected through a transformer T1 to a rectier V1, Which may be, for example, of the two-electrode type. This rectifier is connected across one of the diagonale of a bridge circuit having four arms which include tWo `resistors R1 and R2, which be of the same magnitude, and two additional resistors Ra and R4, R3 vbeing of substantially larger resistance than R1. A condenser C1 is bridged across the resistor R2 and is of large capacity.

The other diagonal of the bridge designated by the points A and B is connected to a second rectifier V2, which may also be, for example, of the two-electrode type as shown. The latter diagonal of the bridge includes, in addition to the rectifier V2, a resistor R5, which is preferably oi' a magnitude larger thanany one of the resistors Ri-RA, inclusive of the bridge. The `resistor R5 is connected to the remainder of the apparatus of the radio receiving set which is schematically represented by the rectangle 2, lthe latter apparatusincluding an audio `frequency amplifier and a loud speaker (not shown).

l A condenser C2 which is interposed between the secondary Winding of the transformer T1 and the anode of the rectifier V1`is a radio frequency coupling condenser, which also prevents any-flow of direct current through the secondary winding of the transformer T1. A condenser C3 is connected lacrossthe points A and B of the bridge, and this condenser shunts the series circuit including the rectifier V2 and the resistor R5. is an audio frequency coupling condenser, which serves also to prevent the flow of direct current from the rectier V to the audio frequency circuits designated 2. The condenser C5 which shunts the primary Winding of the transmformer T1, is a tuning condenser for improving the selectivity of the receiving system.

Each of the resistors R1 and Rz may be, for exarmgile,` of 250,000 ohms. The resistor R3 may be, for example, three times as `large as the resistor R1, the resistor R3 bei`ng,'lets say, 600,000 ohms, and the resistor R4 200,000 ohms. The resistor Rs may be, for example, as large as a megohm. These values are given, of course, merely for the purpose of illustration, and are not to be construed `as limitations upon the invention.

It will be observed vthat the rectifiers V1 and V2 are connected across opposite diagonals of the bridge. The rectifier V1 will rectify all alternating current transmitted through the transformer T1, and the rectified current will be impressed across the diagonal of the bridge to which the rectifier V1 is connected. The rectifier V2 is poled so as to transmit current through the resistor R5 only when the point B of the bridge is at a positive potential with respect to the point A. When the potential of point B 'is either equal to that of point A, or is negative with respect to point A, the rectifier V2 Will block `the oW of all current to the resistor Rs and hence to the audio frequency circuits` 2. Itwill be clear, of course, that when the points A and B are at equal potentials the bridge will then be balanced.

. vAs the resistor Rs is substantially larger than A condenser C4 1 Fig. 1 is not believed to be necessary toa com-` all of the other resistors, it will render the efliciency of detection quite high. All of the resistors shown in Fig. l have an impedance which is manyf times the reactance of thecondenser C1 at the "lowest "modulation frequencies. The theory of operation of the circuit illustrated in plete understanding of the invention, but will be briefly given hereinafter merely to explain the principles involved in the operation of the circuit, it being understood that the theory may not be completely accurate in all respects.

When an unmodulated carrier wave is received by the system and is rectified by the rectifier V1, the rectified voltages existing across resistors R1 andLR2L.wi1l.ibe substantially equal, while the voltage across the resistor R3 will be larger than that across resistor R4, and in the example `given hereinabove,` three times .as large. The potential at vpoint B on` the bridge will be positive with respect' to point A which is grounded. Current will then .l iiow through the rectiiier V2 and throughsits 4associated resistor, R5.

When theincoming carrier wave is modulated bya signal, thelmodulated` wave will have an envelope which will roughly correspond to the Wave shape of the signal current. The modulated current will vary from a small orvnegligible value to a larger value during each cycle of the signal current. The current will be at a minimum value corresponding to a trough in the envelope at one instant, and will reach a peak value-corresponding to a crest in the envelope at another instant, as is well known. If the carrier is onehundred per cent. modulated, the amplitude of the modulated current at a crest will be" approximately twice the magnitude of the unmodulatedv carrier current. Such a modulated current maybe treated as composed of a direct current component and an alternating current component, both components being equal and of the'same polarity at one phase of the moduiatedwave corresponding to a crest in the envelope, these components being equal and of opposite polarity'at another phase of the modulated wave` corresponding to a trough in the envelope. r l

When-'subha modulated'wave is impressed upon the rectifier V1 and rectied thereby, and the rectified current then transmitted to the bridge; the rectified voltages appearing across resistors R1, R3, and R4 will lvary in accordance with thef'modulation. The alternating'current voltage impressed'across the resistor R2, however, will be small due to the fact that' the condenser C1, which'is connected 'thereacross has a very low reactance at the modulation frequencies. Hence the voltages appearing between points A andB ofthe bridge network will vary in accordance with the envelope of the modulated carrier current.

During modulation troughs the point B on the bridge will be at a positive potential with respect to the groundedpoint A, and this po-sitive potential will-be approximately twice that impressed upon point B with respect to point A when only the unmodulated carrieris received. During modulation crests, i. e., that part of the modulation cycle in which the apparent direct current and alternating current components are of equal and, opposite potentials, there will be 11.0.'potentialbetween the points A and B on the bridge and the bridge will attain a state of balance., No'current will ow'either throughthe rectifier V2 or through the resistor R5 during such intervals.

Hence it will be apparent that the modulation cycle will be reproduced across the load resistor R5. However, the' potential across the resistor `R5 may be viewed as 180 degrees out of phase with respect to the incoming modulation cycle.

It will be observed that the bridge is normally unbalanced. That is, as rectified current ows through the bridge a potential appears between points A and B thereof both when the carrier is unmodulated and when the carrier is modulated, except during the brief intervals when the modulated current is at its peak. The bridge becomes balanced only during that part of the cycle when a modulation peak is reached, but this balance is disturbed again immediately after the peak is passed. y

VWhen noise impulses which exceed the amplitude of the fully modulated carrier current are impressed upon the receiving system, the instantaneous values of the voltages existing across the various arms of the bridge are such that the bridge is then momentarily unbalanced. When such noise impulses are received, the point B on the bridge becomes negative with respect to the grounded point A as will be explained hereinafter. During such periods the rectifier V2 will block the flow of current through the resistor R5, and hence noA current will be impressed upon the audio frequency circuits 2. The current due to the noise impulses will not traverse the rectifier V2 for the reason that a negative potential will then be applied to the anode of the rectifier V2, and hence the rectifier will be non-conductive. Thus the rectier V2 will be conductive during the signal interval provided no large noise impulses are present, but when the large noise impulses arrive, the rectifier V2 will block all passage of current therethrough.

Thus the bridge described above is balanced for normal modulation crests in the modulated carrier wave and at that time the voltage across resistor R4. equals that across resistor R2. It becomes progressively unbalanced with 'point' B positive with respect to point A as the modulation envelope falls to the trough and the voltage on R4 thendecreases, the latter voltage approaching Zero. For modulation peaks exceeding the normal crests the voltage across resistor R4 becomes greater than that across resistor R2, point B becomes negative with respect to point A and the bridge is again unbalanced but in the opposite direction.

'Ihe maximum amplitude of the impulses that may reach the vresistor R5 is thus limited to the amplitude corresponding to normal modulation crests in the modulated carrier wave. It will be observed that the voltagefacross resistor R2 and 4-condenser C1 determines the amplitude at which limiting begins. This voltage is established by the unmodulated carrier amplitude and the bridge thus automatically adjusts itself so that noise impulses in the output are limited to the crest Value of normal modulation for the particular carrier amplitude existing. However, the noise impulses above referred to are of very short duration, and as their amplitude may be limited, they will be hardly noticeable in the loud speaker attached to the audio frequency circuits 2.

Fig. 2 of the drawing is in many respects similar to Fig. 1. The rectier V2, however, is reversed so that current will iiowthrough the. rectifier V2 "Lit Cil

of large amplitude which are transmitted thereto y from` the bridge. These noise impulses willappear across theresistor R5, and they will be substantially free of the normal ,modulation fre-` quencies. The voltages appearing across the resistorR will be used to control the output of the vacuum tube detector designated V3 as will be pointed out hereinafter.

Acondenser `Cs is connected between the secondary winding ofthe transformer T1 andthe anode of the vacuum tube V3. This condenser will transmit practically all "of the radio frequencies to the vacuum tube V3, Vthe latter tube acting as a detector for Vthese radio frequencies. The tube V3 maybe, for' example, the second detector'of the radio receiving system.` The rectified currents will' appear in the output circuitof the tubel Vjwhich-includes `the resistor Re, the condenser Cv which `shunts the resistor Re, vand `the radiofrequency choke coil L. A ysource of plate potential "(not shown in`Fig.` 2)v may, if desired, be added to the output circuit in any well known manner. The detected current appearing in the output circuit of tube V3 will be transmitted through the condenser C4 to the audio frequency amplifier `3, `which may include a loud speaker (not shown)v attached thereto.

As noise impulses of large- 'amplitude are rectied by the rectifier V2 and appear across the resistor R5, they will' be applied to the grid of the tube V3 and render the` gridv highly negative with respect to' the associated cathode of the tube. The negative voltage "so applied to the grid will limit or vcut off the current owing 'through the plate circuit of the tube V3 for the `duration of suchv noise impulses. 'I'he output ofthe tube Va will, therefore, not only greatly limit the amplitude of the noiselimpulses but may remove them altogether when the ratio of the noise-to-carrier amplitude is suicie'ntly large. The noise impulses are separated from the signal impulses by an arrangement of Fig. 2 and the noise impulses are employed to control the action of the rectifying tube V3 and limit or cut off the current nowing in the plate circuit when the grid becomes actuated by these impulses. c

Referring to Fig..3 of the drawing, the instantaneous voltage appearing across points A and B of the bridge network will be applied to the grid ofthe audio frequency amplifier V4 through the` resistor R5, the resistor R5 acting asa volume regulating potentiometer. The grid of the tube V4 is biased quiteaccurately to the cut-01T point when no signals are being received. This is accomplished by making the. cathode of the .tube lpositivebothwith respect-to ground and with respect tof the grid of the tube. The voltage divider R1 isusedfor this purpose, the element Rv being connected across thevplate voltage supply' P. By this arrangement the cathode bias is` quite correctly proportioned, even 'though the plate voltage may' iiuctuate regulation.

The voltage divider Rv should preferably be rof comparatively low resistance so that 'the flow of cathode currentresulting'from the amplification of signalswill not `appreciably change thetbias. The impedance in theplatecircuit ofthe tube`V4 on account of poor should c also have` a comparatively; low direct curi rent.

directcurrent'potential applied tothe plate electrodeof the tube. i

When an unmodulated carrier current isbeing received bythe system the Idirect current potentialimpressed between points AA and B of the` bridge network will render the grid of the tube V4 more positive with respect tothe cathode. Hence plate current will flow through the tube V4. When the carrier current becomes modulated bysignals the bridge vcircuitwill reverse the a1- ternating current component of these signals as already pointed out hereinabove. 'Ihe grid of the tube V4 Willrbecome more negative when the envelope of themodulated wave' reaches a modulation peak and less negative as the envelope passes .through a modulation trough. The voltage applied-to the gridvwill thusA vary between the cut-oir value and some lvalue at which a largevv -rentwill now through the tube V4. i During such intervalsthere will berno signal reproduction by the tube V4.

'Thus as the complete modulation cycle is impressed upon thegrid of the tube V4 as apositive voltage, plate `current will normally ow` through the `tube and the modulation cycle will be' faithfully `amplified and reproduced in the amplifier output. The accuracy of the reproductionfwill depend somewhatupon the linearity of thel grid-voltage plate-current characteristic `curve ofthe tube in the region between the zero grid bias value and the cut-olf point. This latter requirement can be easily met `in practice. The noise impulses which are of large ampli- `tudemerely act to increase'the bias of thegrid of the tube V4 momentarily. The noise impulses 4will then be rejected rby the tube V4, even though no signal is being received by the system.

`When a signal is being received, however, the amplitude of the noise impulses will not exceed the crest value-'of 'the normal modulated cur- 'I'he circuit just described will substantially reduce, if not entirely eliminate from the output uof the system, the impulses generated by ignition systems and the like.

The bias for the cathode ofA the tube V4 is quite critical. It has been determined that insuflicient bias reduces the noise reduction ability Aof the system lsomewhat, while an` excessive bias introduces distortion. The bias to the cathode should preferably be set at a` value where the .plate current willV cease to ow when no signal 'is being impressed upon the tube V4. Oncethis bias `is correctly established it need not be disturbed again.

The condenserfCs may be added to the circuit in shunt with the secondary winding of the transformer T1 for the purpose of increasing the selectivity of the receiving system,` if this be desired. The condenserV C9, which is connected be- ,tweenthe cathode of the `tube V4 and ground, is of such capacity as to act as a by-pass for audio frequency current. The condenser C10 whichbridges the source of-plate voltage Pfis similarly employed to act as a by-pass for current of audio frequencies.

While this invention has been shown and described in certain particular arrangements merely for the `p urpose of illustration, it Will be understood that the general principles of` this invention may be applied to other and Widely varied organizations Without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is: Y v H1. Apparatus forv receiving modulated signals free from noise impulses, comprising the combination of a bridge circuit, said bridge circuit including four resistors and a condenser in shunt with one of said resistors, two rectiersconnected respectively across the opposite diagonals of the bridge circuit, means for impressing the modulated signals upon one oi said rectifiers, and means for translating the signals impressedl by the bridge circuit upon the other rectifier into currents which are substantially free from noise impulses. l

2. A radio receiving system for translating received radio frequency currents into audible signals substantially free from noise impulses that may be impressed simultaneously With the signals upon the radio receiving system, comprising a first rectifier for rectifying the received currents, a bridge circuit, said first rectier being connected as a diagonal of said bridge circuit, a second rectifier connected to the other diagonal of the bridge circuit, said second rectier being biased so as to block the passage of all currents therethrough when noise impulses are impressed upon the bridge circuit, and means for translating the signals traversing said second rectifier when substantially no noise impulses are impressed upon the system.

3. A radio receiving system for translating received radio frequency currents into audible signals substantially free from noise impulses, comprising a detector which prevents the iiow of current therethrough when the polarity of said currents is in one direction and recties said currents When their polarity is in the opposite direction, a bridge circuit connected to said detectorso that the detector is a diagonal thereof, said bridge circuit comprising four resistors which form the arms thereof and a condenser which shunts one of said arms, means including said bridge circuit for applying to said detector currents oi one polarity when the signals received by the system are substantially free from noise impulses and for impressing upon said detector currents of the opposite polarity when substantial noise impulses are impressed upon the system.

4. In a radio receiving system for discriminating against noise impulses of large amplitude, means for rectifying all of the currents impressed upon the system, a bridge circuit one of the diagonale of which is supplied With the rectified current, a detector, a resistor connected in series with said detector across the opposite diagonal of the bridge circuit so that all of the impulses detected by said detector will traverse said resistor, and an amplifying device connected across said resistor for amplifying only those voltages appearing across said resistor.

5. In a radio receiving system for discriminating against noise impulses of large magnitude, comprising a bridge'circuit, one diagonal of which is supplied With all of the currents impressed upon the receiving system, -a three-electrode vacuum tube the grid and lament of which are connected to the other diagonal of the bridge circuit, means responsive to the signal currents for varying the potential impressed by the bridge circuit upon the grid of said tube so that said currents ymay be translated by the tube, and means responsive to noise impulses of large magnitude transmitted by the bridge circuit to the gridoof the tube for so biasing the grid of the tube as to prevent the iiow of anode current through said tube While said noise impulses are present.

y `6. A radio receiving system upon which may be impressed radio frequency signals and noise impulses, a detector tube connected as part of said radio receiving system, said tube having grid, filament and plate electrodes, means for impressing voltages on the grid of said tube corresponding to the signals received by the system so that said tube may detect said signals, means responsive to the noise impulses that may be simultaneously impressed upon the system for rendering said detector tube inoperative during the interval when said noise impulses are present, said latter means comprising a bridge circuit which is connected ahead of said detector tube and as a part of said receiving system, said bridge circuit. including four non-inductive resistors as arms thereof and a condenser in shunt With one of said arms.

7. The combination of means for receiving signal currents as Well as noise impulses, a Wheatstone bridge circuit to which said receiving means is connected as one diagonal thereof, detecting means connected as the other diagonal of said bridge circuit, means responsive to ysaid signal currents for applying through said bridge circuit voltages of one polarity for detection by said detecting means, and means responsive to said noise impulses for applying voltagesv of the opposite polarity through said bridge circuit to said detecting means to render said detecting means inoperative.

8. Apparatus for producing a potential of one polarity when a received alternating voltage exceeds a predetermined value and a potential of opposite polarity when the alternating voltage is less than said predetermined value, comprising a rectifier for rectifying said alternating voltage, a Wheatstone bridge circuit to which said rectiner is connected as one diagonal thereof, said bridge circuit comprising four resistive arms and a condenser shunting one of said resistive arms, the potentials Vof opposite polarities appearing across the other diagonal of the bridge.

9. In a receiving system for receiving carrier current modulated by signals and for discriminating between carrier current which is normally modulated and excessive currents due to abnormal modulation, comprising means for rectifying the received currents, and means for automatically producing a potential of one polarity When normally modulated current is rectied and a potential of opposite polarity When excessive currents due to abnormal modulation are rectiiied, said latter means comprisingy four nonadjustable non-inductive resistors arranged in Wheatstone bridge relationship and a condenser bridging one of said resistors, the rectified currents being supplied to said bridge circuit, the potentials of opposite polarity being obtained from said bridge circuit.

EDWARD HENRY WEBER, JR,

DISCLAIMER 2,208,958.-Edwad Henry Weber, J 1'., Toms River, N. J. NOISE REDUCING RECEIVING SYSTEM. Patent dated July 23, 194:0. Disclaimer filed August 7, 1941, by the inventor; the assignee, Amewleom Telephone and Telegraph Uompcmy, consenting. Hereby enters this disclaimer to claims 2, 4, and 7 of said Letters Patent.

[Oficial Gazette September Z, 1.941.] 

